miri: Cannot build "backtrace" with locally-compiled rustc on macOS: expected enum `std::os::raw::c_void`, found enum `libc::c_void`

I locally built rustc master and linked stage2 using rustup, set the override in the miri subdir, but I’m getting this when running cargo build:

$ cargo build
   Compiling backtrace v0.3.29
   Compiling serde v1.0.92
error[E0308]: mismatched types
   --> /Users/alex/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.29/src/symbolize/coresymbolication.rs:155:32
    |
155 |                       self.dll = libc::dlopen(lib.as_ptr() as *const _, libc::RTLD_LAZY);
    |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected enum `std::os::raw::c_void`, found enum `libc::c_void`
...
186 | / coresymbolication! {
187 | |     #[load_path = "/System/Library/PrivateFrameworks/CoreSymbolication.framework\
188 | |                  /Versions/A/CoreSymbolication"]
189 | |     extern "C" {
...   |
202 | |     }
203 | | }
    | |_- in this macro invocation
    |
    = note: expected type `*mut std::os::raw::c_void`
               found type `*mut libc::c_void`

error[E0308]: mismatched types
   --> /Users/alex/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.29/src/symbolize/coresymbolication.rs:176:39
    |
176 |                       match libc::dlsym(self.dll, symbol.as_ptr() as *const _) as usize {
    |                                         ^^^^^^^^ expected enum `libc::c_void`, found enum `std::os::raw::c_void`
...
186 | / coresymbolication! {
187 | |     #[load_path = "/System/Library/PrivateFrameworks/CoreSymbolication.framework\
188 | |                  /Versions/A/CoreSymbolication"]
189 | |     extern "C" {
...   |
202 | |     }
203 | | }
    | |_- in this macro invocation
    |
    = note: expected type `*mut libc::c_void`
               found type `*mut std::os::raw::c_void`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0308`.
error: Could not compile `backtrace`.
warning: build failed, waiting for other jobs to finish...

Note that ./x.py build src/tools/miri works just fine.

I’ve discussed this a bit with @RalfJung on chat but not had much luck, so I thought I’d post it here so we can treat it as a proper issue.